In traditional tachometers, engine RPM (revolutions per minute) is converted to a representative voltage by feeding pulses from the ignition system to a frequency-to-voltage converter circuit. Every pulse from the ignition adds a fixed electrical charge to the driver circuit; the faster the engine runs, the faster the ignition pulses occur, and the more charges are pumped into the driver circuit. A driver circuit causes the pointer to move at an angle proportional to the rate that the charges occur.
There are many known tachometers. Some of which are discussed below. For example, U.S. Pat. No. 5,004,979 to Marino et al. describes a battery tachometer, where the input signal is from the battery, which is related to the RPM of the engine.
U.S. Pat. No. 5,182,512 to Braun et al. describes an engine analyzer (handheld tachometer) designed to be held in contact with a fan belt of an engine to determine cylinder power balance (from engine speed variations). That is, the handheld tachometer is used for determining the relative power contributions of a multi-cylinder internal combustion engine. The engine analyzer is used for cars, which have a different number of pulses per revolution.
U.S. Pat. No. RE38,100 to Rentsch et al. describes a tachometer apparatus including a microphone for disposition next to the exhaust and an accelerometer next to the engine. That is, the apparatus is an external device that calculates and which displays the speed of the engine based on sensing engine vibrations. The tachometer has two channels, where one channel is used when the engine is idle and another channel is used when the engine is in normal operation.
U.S. Pat. No. 4,705,969 to Gross describes a tachometer circuit that has a zero ripple DC output that is directly proportional to the frequency. The circuit uses two full wave rectifiers to act on the sine and cosine signal inputs. The signal is rectified, squared, summed and extracted.
U.S. Pat. No. 5,883,510 to Torre et al. describes a ground pick-up tachometer, separate from the vehicle ground. The '510 patent describes a noise threshold, but does not remove flutter.
U.S. Pat. No. 4,059,799 to Faria describes a tachometer apparatus for use on automobile assembly lines and accepts an electric pulse input of frequency, which is directly related to engine speed. The purpose is to measure speed of engines with different numbers of cylinders on an assembly line. The pulsating input signal is characterized by pulses of alternating polarity and uses a rectifier with shunts to measure the speed of the engines passing through the assembly line.
U.S. Pat. No. 3,978,719 to Hadden et al. describes an external apparatus including a pressure transducer for sensing pressure variations at the exhaust of a vehicle. That is, the apparatus is external to the engine and the signal sensed is passed through an amplifier with automatic gain control and then through a tracking filter to separate the prominent frequency component.
U.S. Pat. No. 3,934,200 to Schoonover et al. describes an apparatus for producing a non-linear ripple-free output voltage using a temperature compensation circuit, a ripple reduction circuit, a linearity compensation circuit, a summing amplifier circuit and a low RPM linearity compensation circuit.
U.S. Pat. No. 3,893,014 to Vacek describes a system whereby ripple is removed independent of generator speed using two tachometer generators where both signals are full wave rectified to eliminate the fundamental.
U.S. Pat. No. 3,886,450 to Trusell et al. describes an apparatus for measuring the RPM of an internal combustion engine without attachment to the engine.
U.S. Pat. No. 3,822,402 to Vest describes producing a voltage across a meter that varies essentially linearly with engine speed. 
U.S. Pat. No. 3,662,252 to Smith describes a tachometer including an alternator connected to a shaft which generates an alternating voltage.
U.S. Pat. No. 3,662,251, “Method and System for Measuring Acceleration and Velocity” by Smith describes the simultaneous generation of acceleration and velqcity signals.
U.S. Pat. No. 3,636,451 to Westberg describes an apparatus for use with engines having a different number of cylinders. The apparatus uses a relay switch and a relay coil.
U.S. Pat. No. 3,614,616 to Bucek et al. describes an AC tachometer and circuit in which an AC tachometer generator has two out-of-phase windings independently connected through rectifiers with outputs connected in parallel to provide a DC output potential indicative of the rate of rotation.
U.S. Pat. No. 3,586,949 to Spear et al. describes a tachometer that is used to generate a signal indicative of the motor speed which is combined with a command signal and a current limit signal.
Traditional tachometers, however, suffer from a visible pointer flutter at low pulse rates. This situation is aggravated by the 1 and ½ pulse per revolution ignitions typically used in motorcycles. Traditional solutions have been ineffective and cause other problems (e.g., they reduce flutter at the expense of slowing down the response of the pointer when there are rapid changes in the engine RPM).
Accordingly, it is desirable to reduce flutter in a tachometer without adversely affecting the response of the tachometer.